Materials for packaging foods and various articles often are required to have gas barrier properties represented by oxygen barrier properties. The use of a packaging material having low gas barrier properties may sometimes cause deterioration of the contents due to the decay of foods, etc., resulting from oxidation by oxygen or proliferation of microorganisms. Therefore, conventional packaging materials generally include a gas barrier layer for preventing the permeation of oxygen, etc.
Examples of the gas barrier layer include metal foils, and vapor deposition layers of metals or metal compounds. Aluminum foil, aluminum vapor deposition layer, silicon oxide vapor deposition layer, and aluminum oxide vapor deposition layer, for example, are used for the gas barrier layer. However, aluminium foil and metal layers such as aluminium vapor deposition layer have a disadvantage of invisibility of the contents or a disadvantage of difficulty in disposal. Meanwhile, metal compound layers such as silicon oxide vapor deposition layer and aluminium oxide vapor deposition layer may have cracks in the compound layer in some cases when the packaging material is deformed or subjected to impact, thus resulting in significant deterioration in gas barrier properties.
Also, a layer composed of a vinyl alcohol polymer with excellent gas barrier properties (e.g., polyvinyl alcohol and ethylene-vinyl alcohol copolymer) may be used as a gas barrier layer in some cases. Such a layer composed of the vinyl alcohol polymer has advantages of transparency and less difficulty in disposal.
The above-mentioned vinyl alcohol polymer is crystallized and densified by hydrogen bonding between hydroxyl groups in the molecule to exert gas barrier properties. Therefore, the hydrogen bonds of the conventional vinyl alcohol polymer are weakened and the gas barrier properties thereof tend to deteriorate in a state where the polymer has absorbed moisture under the influence, for instance, of water vapor, though it exhibits high gas barrier properties in a dry state. Accordingly, it is difficult for the layer composed of the vinyl alcohol polymer to exert a high level of gas barrier properties under high humidity.
Further, materials containing a polymer compound and a hydrolyzed condensate of metal alkoxide (for instance, tetramethoxysilane) are disclosed as materials for gas barrier layers (e.g., JP 2002-326303 A, JP 7 (1995)-118543 A, and JP 2000-233478 A). Furthermore, a gas barrier layer composed of polyacrylic acid and a crosslinking component is disclosed (e.g., JP 2001-310425 A).
In recent years, retort foods in which packaging materials filled with contents (foods) are brought into contact with hot water for sterilization have become widespread. In such a situation, the level of performance required for packaging materials for retort foods, such as strength against bag-breakage when dropping a packaging material filled with contents, oxygen barrier properties after sterilization by contact with hot water, oxygen barrier properties under high humidity until the delivery to a consumer, is becoming higher. The above-mentioned conventional techniques, however, cannot satisfy such requirements sufficiently.
A method for solving the above-mentioned problems and improving the properties of a gas barrier layer dramatically has been proposed (WO 2005/053954 A1). In this method, a gas barrier layer formed of a composition that includes a hydrolyzed condensate of metal alkoxide and a —COO— group-containing polymer is immersed in a solution that contains a metal ion having a valence of at least two. The —COO— group in the polymer is neutralized by this treatment.